Contrary to popular belief "mouth fucking" or "throat fucking" is not simply a blow-job.
Where as in a blow-job the one giving it is doing the majority of the work and technique is involved, in mouth fucking the person getting their dick wet is doing the work with pelvic thrusts while the person getting mouth fucked simply lets it happen trying their hardest not to gag.
Blow-jobs are far more common than mouth fucking as it is harder to find a partner willing to let you literally fuck their mouth/throat.
"Man that girl is a real freak, not only does she like to suck dick, she likes to get mouth fucked!"
Guy 1 : "You know what really gets me off?"
Guy 2 : "What?"
Guy 1 : "Mouth fucking!"
Guy 2 : "Damn dude that's narly!"